Method for heating glass sheet, and glass tempering furnace

ABSTRACT

A glass tempering furnace and a method for heating a glass sheet. The glass sheet is heated in the glass tempering furnace by blowing heating air on the top surface of the glass sheet, and the blowing distance of the heating air from the top surface of the glass sheet is adjusted.

BACKGROUND OF THE INVENTION

The invention relates to a method for heating glass sheets, and glasstempering furnace.

As glass sheets are heated in a glass tempering furnace, the aim is toheat them as evenly as possible. Any unevenness in the temperature of aglass sheet will result in tension in it and consequently optical errorsin the glass. To establish as even as possible a temperature effect, theaim is to diversely adjust the temperature profile of the glass sheet.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide a new type of methodfor heating glass sheets and a new type of glass tempering furnace.

The solution of the invention is characterised by what is disclosed inthe independent claims. Some embodiments of the invention are disclosedin the dependent claims.

In the solution put forth, a glass sheet is led into a glass temperingfurnace, the glass sheet is heated in the glass tempering furnace by atleast blowing heating air on the top surface of the glass sheet, and theblowing distance of the heating air from the top surface of the glasssheet is adjusted. By adjusting the blowing distance of the heating air,it is possible to influence the thermal effect that the heating airdirects on the top surface of the glass sheet. The blowing distance maybe adjusted either before the glass sheet is fed in the glass temperingfurnace or when the glass sheet is already within the tempering furnaceand the glass sheet is being heated by blowing heating air on the topsurface of the glass sheet. Adjusting the blowing distance of theheating air together with adjusting the blowing force of the heatingair, in use already previously, allow a more diverse heating of theglass sheet than before by the use of heating air in such a manner thatthe thermal effect according to the manufacturing formula of the glasssheet can be directed on the glass sheet. The adjustment of the heatingair blowing distance together with the adjustment of the blowing forcealso make it easier to achieve the optimal energy-efficient workingpoint of the tempering furnace, insofar as the use of heating air toheat a glass sheet is concerned.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described in closer detail in connection withpreferred embodiments, with reference to the accompanying drawings, inwhich:

FIGS. 1 and 2 show a schematic sectional end view of a glass temperingfurnace,

FIG. 3 is a schematic sectional end view of a second glass temperingfurnace, and

FIGS. 4, 5, and 6 show a schematic cross-sectional detail of a glasstempering furnace as seen from an end of the glass tempering furnace.

For the sake of clarity, the figures show some embodiments of theinvention in a simplified manner. In the figures, like referencenumerals identify like elements.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 show a schematic cross-sectional end view of a glasstempering furnace 1. The tempering furnace 1 has a frame structure 2which comprises a top part 3 and a bottom part 4, which are separatefrom one another in the sense that the top part 3 may be moved in thevertical direction of the tempering furnace 1 away from the bottom part4 and towards it. FIG. 1 shows the tempering furnace 1 in an operatingsituation where the top part 3 of the tempering furnace 1 is in contactwith the bottom part 4. FIG. 2, on the other hand, shows the temperingfurnace 1 in an operating situation where the top part 3 of thetempering furnace 1 has been lifted up, whereby the top part 3 and thebottom part 4 of the tempering furnace 1, in the vertical direction ofthe tempering furnace 1, are separated from one another and there is agap 21 between them. There are means in connection with the gap 21 toprevent the flowing of the blast air used for heating the glass sheet 6out of the tempering furnace 1. The means in question will be examinedin closer detail in connection with FIGS. 4, 5, and 6.

The tempering furnace 1 further has rolls 5, supported in connectionwith the frame structure 2, in FIGS. 1 and 2, in connection with thebottom part 4 of the frame structure 2. Typically, the rolls 5 areceramic rolls 5, forming a conveyor of the tempering furnace 1, by whichthe glass sheets 6 are conveyed to the tempering furnace 1 and out ofit. At the time the glass sheet 6 is being heated, the rolls 5 aretypically driven in such a manner that the glass sheet 6 is oscillatedback and forth in the tempering furnace 1 by the conveyor formed by therolls 5. For reasons of clarity, the figures do not show means known perse for a person skilled in the art to rotate, drive or control the rolls5. Instead of the conveyor formed by the rolls 5, the tempering furnace1 may also make use of other conveying methods to carry the glass sheets6.

The tempering furnace 1 further has blowing channels 7 of the top part3, positioned in the space restricted by the top part 3, which arearranged to blow warm or hot air, that is, heating air, on the topsurface of the glass sheet 6 in order to heat the glass sheet 6.Advantageously, there are several successive said blowing channels 7 inthe direction of travel of the glass sheet 6, that is, in thelongitudinal direction of the tempering furnace 1. For reasons ofclarity, FIGS. 1 and 2 do not show the heating resistors 22, 23 or othermeans to be disclosed below in FIG. 3 that are possibly included in thetempering furnace 1 to warm or heat air, such means being placeable onthe inside or outside of the tempering furnace 1 and being known per sefor a person skilled in the art. The blowing channels 7 in the top part3 are arranged substantially transverse in relation to the direction oftravel of the glass sheets 6. In this context, the definitionsubstantially transverse means in an embodiment that the blowingchannels 7 in the top part 3 are at a 70-110 degree angle with respectto the direction of travel of the glass sheets 6. According to anotherembodiment, the blowing channels 7 in the top part 3 are at an 80-100degree angle with respect to the direction of travel of the glass sheets6. According to yet another embodiment, the blowing channels 7 in thetop part 3 are at an 85-95 degree angle with respect to the direction oftravel of the glass sheets 6.

The heating air is fed to the blowing channel 7 of the top part 3through a feeding channel 8 of the top part 3. Each blowing channel 7may comprise its own, separate feeding channel 8, or at least twoblowing channels 7 may have a feeding channel 8 at least partly common.The tempering furnace 1 further has a blower 9 of the top part 3, usedto feed air to the feeding channel 8. Each feeding channel 8 may beassociated with its own, separate blower 9, or at least two feedingchannels 8 may have a common blower 9. Air is fed back to the blower 9from the top part 3 of the tempering furnace 1 through the returnchannel 10 in the top part 3, whereby the heating air of the glass sheet6 may be circulated in the top part 3 of the tempering furnace 1 by theblower 9. At a top part 7′ of the blowing channel 7 there is a blowingchannel 7 feed part 11, which is wider at its forward end, that is, atthe end of the feeding channel 8, and becoming narrower towards the endin the direction of the heating air flow. This way, heating air can befed evenly along the entire length of the blowing channel 7. At a bottompart 7″ of the blowing channel 7 there are nozzles 12 by means of whichthe flow may be directed on the top surface of the glass sheet 6. Thenozzles 12 may be elongated tubular channels. With such elongatednozzles, the air flow can be effectively and precisely made to reach thedesired place, even from a somewhat longer blowing distance. The blowingforce of the heating air blown through the blowing channel 7 may beadjusted by a control unit 13, which is arranged to control theoperation of the blower 9. Said control unit 13 may be an inverter, forexample, by means of which the running speed of the blower, and hencethe flow rate produced by the blower, are controlled.

The tempering furnace 1 further has blowing channels 14 of the bottompart 4, positioned in the space restricted by the bottom part 4, whichare arranged to blow heating air on the bottom surface of the glasssheet 6 in order to heat the glass sheet 6. Advantageously, there areseveral successive said blowing channels 14 in the direction of travelof the glass sheet 6, that is, in the longitudinal direction of thetempering furnace 1. The blowing channels 14 in the bottom part 4 arearranged substantially transverse in relation to the direction of travelof the glass sheets 6. In this context, the definition substantiallytransverse means in an embodiment that the blowing channels 14 in thebottom part 4 are at a 70-110 degree angle with respect to the directionof travel of the glass sheets 6. According to another embodiment, theblowing channels 14 in the bottom part 4 are at an 80-100 degree anglewith respect to the direction of travel of the glass sheets 6. Accordingto yet another embodiment, the blowing channels 14 in the bottom part 4are at an 85-95 degree angle with respect to the direction of travel ofthe glass sheets 6.

The heating air is fed to the blowing channel 14 of the bottom part 4through the feeding channel 15 of the bottom part 4. Each blowingchannel 14 may comprise its own, separate feeding channel 15, or atleast two blowing channels 14 may have a feeding channel 15 at leastpartly common. The tempering furnace 1 further has a blower 16 of thebottom part 4, used to feed air to the feeding channel 15. Each feedingchannel 15 may be associated with its own, separate blower 16, or atleast two feeding channels 15 may have a common blower 16. Air is fedback to the blower 16 from the bottom part 4 of the tempering furnace 1through the return channel 17 in the bottom part 4, whereby the heatingair of the glass sheet 6 may be circulated in the top part 4 of thetempering furnace 1 by the blower 16. At a bottom part 14′ of theblowing channel 14 there is a blowing channel 14 feed part 18, which iswider at its forward end, that is, at the end of the feeding channel 15,and becoming narrower towards the end in the direction of the heatingair flow. This way, heating air can be fed evenly along the entirelength of the blowing channel 14. At a top part 14″ of the blowingchannel 14 there are nozzles 19 by means of which the flow of theheating air may be directed on the bottom surface of the glass sheet 6.The nozzles 19 may be elongated tubular channels, either similar to ordifferent from the ones in the blowing channels 7 of the top part 3. Theblowing force of the heating air blown through the blowing channel 14may be adjusted by a control unit 20, which is arranged to control theoperation of the blower 16. The control units 13 and 20 may be one andthe same physical device.

FIGS. 1 and 2 show a possible structure for a blowing channel 7, 14 usedin the tempering furnace 1. The structure of the blowing channels 7, 14of the tempering furnace 1 may also differ from the one presented inFIGS. 1 and 2. So, the nozzles 12, 19 of the blowing channel 7, 14, forexample, may be replaced by, for example, a nozzle plate, such as aperforated plate, or a channel structure entirely open towards the topand/or bottom surface of the glass sheet 6. The tempering furnace 1shown in FIGS. 1 and 2 may have, in its top part 3 in addition to theblowing channels 7, also other means for heating the glass sheet 6 fromabove. The bottom part 4 of the tempering furnace 1 may, either inaddition to or instead of the blowing channels 14, have other means forheating the glass sheet 6 from below. The rolls 5 may also be used toheat the glass sheet 6 from below by, for example, arranging means inconnection with the rolls 5 for heating the rolls 5.

The blowing channels 7 in the top part 3 of the tempering furnace 1 arefixedly supported to the structure of the top part 3 of the temperingfurnace 1. In such a case, when the top part 3 of the tempering furnace1 is being moved in the vertical direction in relation to the bottompart 4 of the tempering furnace 1 in the direction schematically shownby arrow A in FIG. 2 away from the bottom part 4 or towards it, theblowing channels 7 in the top part 3 move along with the top part 3 awayfrom the bottom part 4, or towards it, whereby the distance of theblowing channels 7 in the top part 3 from the glass sheet 6, in otherwords, the blowing distance D of the heating air from the glass sheet 6,will change. In the operating position according to FIG. 1 of thetempering furnace 1, where the top part 3 is supported to the bottompart 4, the blowing distance D is at its minimum. The blowing distance Dis at its maximum when the top part 3 is moved to the maximum distancefrom the bottom part 4, allowed by the adjustment range between the toppart 3 and the bottom part 4. The top part 3 may be moved in relation tothe bottom part 4 by, for example, a motor or another actuator and apower train mechanism arranged between the actuator and the temperingfurnace 1.

So, when the position of the top part 3 of the tempering furnace 1 ischanged in relation to the bottom part 4 in the vertical direction ofthe tempering furnace 1, the distance of between the blowing channels 7in the top part 3 and the glass sheet 6 is changed at the same time, inother words, the blowing distance from the glass sheet 6 of the heatingair directed on the glass sheet 6 is adjusted. By adjusting the blowingdistance D of the heating air, it is possible to influence the thermaleffect that the heating air directs on the top surface of the glasssheet 6. The position of the top part 3 of the tempering furnace 1 inrelation to the bottom part 4 may be changed either before the glasssheet 6 is fed in the tempering furnace 1 or when the glass sheet 6 iswithin the tempering furnace 1. Adjusting the blowing distance D of theheating air together with the presented adjusting of the blowing forceof the heating air allow a more diverse heating than before of the glasssheet 6 by heating air in such a manner that the thermal effectaccording to the manufacturing formula of the glass sheet 6 can bedirected on it. The adjustment of the heating air blowing distance Dtogether with the adjustment of the blowing force also make it easier toachieve the optimal energy-efficient working point of the temperingfurnace 1, insofar as the use of heating air to heat the glass sheet 6is concerned.

The distance of the blowing channels 7 in the top part 3 of thetempering furnace 1 from the glass sheet 6 may also be adjusted byarranging the blowing channels 7 in connection with the top part 3 ofthe tempering furnace 1 in a movable manner so that the position of theblowing channels 7 in the vertical direction of the tempering furnace 1in its top part 3 may be changed without moving the top part 3 of thetempering furnace 1 in relation to the bottom part 4. In such a case,the thermal effect, directed by the heating air blown towards the glasssheet 6 through the blowing channels 7, on the glass sheet 6 may beadjusted by changing the distance of the blowing channels 7 to the glasssheet 6 without the top part 3 of the tempering furnace 1 being moved inrelation to the bottom part 4. This also makes it possible toindividually change the distance of the blowing channels 7, which areplaced successively in the longitudinal direction of the temperingfurnace 1, from the glass sheet 6, whereby the successive blowingchannels 7 in the longitudinal direction of the tempering furnace 1 mayalso be set, if so desired, at different distances from the glass sheet6.

Also such an embodiment is possible where both the position of theblowing channels 7 in the top part 3 of the tempering furnace 1 and theposition of the top part 3 of the tempering furnace 1 in relation to thebottom part 4 may be changed.

FIG. 3 is a schematic cross-sectional end-view representation of asecond glass tempering furnace 1. The basic structure of the temperingfurnace 1 shown in FIG. 3 is similar to what is shown in FIGS. 1 and 2,that is, the tempering furnace 1 has a frame structure 2 which comprisesa top part 3 and a bottom part 4, which may be separated from oneanother, so the top part 3 of the tempering furnace 1 may be moved inrelation to the bottom part 4 in the vertical direction of the temperingfurnace 1, as schematically shown by arrow A in FIG. 2. FIG. 3 shows thetempering furnace 1 in an operating situation where the top part 3 ofthe tempering furnace 1 is in contact with the bottom part 4. Thetempering furnace 1 shown in FIG. 3 additionally has blowing channels 7in the top part to blow heating air on the top surface of the glasssheet 6, and blowing channels 14 in the bottom part 4 to blow heatingair to the bottom surface of the glass sheet 6.

The top part 3 of the tempering furnace 1 further has several heatingresistors 22 placed one after the other in the direction of travel ofthe glass sheet 6. The bottom part 4 of the tempering furnace 1, too,has several heating resistors 23 placed one after the other in thedirection of travel of the glass sheet 6. Just like the blowing channels7, 14, also the heating resistors 22, 23 are arranged substantiallytransverse in relation to the direction of travel of the glass sheets 6.In this case, too, the definition substantially transverse in relationto the direction of travel of the glass sheets 6 means that in anembodiment the heating resistors 22, 23 are at a 70-110 degree anglewith respect to the direction of travel of the glass sheets 6. Accordingto another embodiment, the heating resistors 22, 23 are at an 80-100degree angle with respect to the direction of travel of the glass sheets6. According to another embodiment, the heating resistors 22, 23 are atan 85-95 degree angle with respect to the direction of travel of theglass sheets 6.

In the embodiment shown in FIG. 3, the heating resistors 22 in the toppart 3 of the tempering furnace 1 are placed, in the vertical directionof the tempering furnace 1, below the blowing channels 7, between theglass sheet 6 and the blowing channels 7, when the blowing channels 7are in the closest possible position in relation to the glass sheet 6.The blowing channels 7 and the heating resistors 22 may be overlappingor they may be aligned in the direction of travel of the glass sheets 6.Deviating from the embodiment of FIG. 3, if the blowing channels 7 andthe heating resistors 22 are aligned in the direction of travel of theglass sheets 6, the blowing channels 7 may be placed, in the verticaldirection of the tempering furnace 1, at such a distance in relation tothe glass sheet 6 that the heating resistors 22 are sited inside theblowing channels 7 when the blowing channels 7 are at the minimumdistance from the glass sheet 6, as defined by the adjustment range ofthe distance adjustment between the blowing channels 7 and the glasssheet 6. The heating resistors 22 may be placed inside the blowingchannels 7 also in such a way that the heating resistors 22 aresupported to the blowing channels 7 so that they will stay within theblowing channels 7 in the entire adjustment range of the distanceadjustment between the blowing channels 7 and the glass sheet 6.

In the embodiment shown in FIG. 3, the heating resistors 23 in thebottom part 4 of the tempering furnace 1 are placed, in the verticaldirection of the tempering furnace 1, below the rolls 5. The blowingchannels 14 and the heating resistors 23 may be overlapping or they maybe aligned in the direction of travel of the glass sheets 6. Theembodiment of FIG. 3 assumes that the blowing channels 14 and theheating resistors 23 are aligned in the direction of travel of the glasssheets 6, whereby the heating resistors 23 are placed inside the blowingchannels 14.

If the heating resistors 22, 23 are arranged outside the blowingchannels further from the glass sheet 6 to be heated than the blowingchannels 7, 14, an adequately wide gap needs to be left betweensuccessive blowing channels 7, 14 in the longitudinal direction of thetempering furnace 1 so that the radiation from the resistors couldefficiently heat the glass sheets.

The heating resistors 22 in the top part 3 of the tempering furnace 1may be used to directly heat the glass sheet 6 from above. The heatingresistors 23 in the bottom part 4 of the tempering furnace 1 may be alsoused to directly heat the glass sheet 6 from below. In addition to orinstead of this, the heating resistors 22, 23 may also be used to heatthe heating air blown from the blowing channel 7, 14. The heatingresistors 22, 23 are used to heat the heating air blown from the blowingchannel 7, 14 in particular in case the heating resistors 22, 23 areplaced inside the blowing channels 7, 14. The heating resistors 22, 23may be used for both direct heating of the glass sheet 6 and for heatingthe heating air blown from the blowing channels 7, 14 when the heatingresistors 22, 23 are placed in alignment with the blowing channels 7, 14in the direction of travel of the glass sheet 6, between the blowingchannels 7, 14 and the glass sheet 6.

The use of the heating resistors 22 together with blowing heating airfrom the blowing channels 7 accomplishes, together with the blowingforce and blowing distance of the heating air, a third dimension toadjusting the heating of the glass sheet 6 in the top part 3 of thetempering furnace 1, whereby the blowing force and blowing distance ofthe heating air used to heat the glass sheets 6 as well as the power ofthe heating resistors 22 may be simultaneously adjusted in order toachieve the optimal working point of the tempering furnace 1 when theglass sheet 6 according to the manufacturing formula is manufactured.

In the embodiment of FIG. 3, each heating resistor 22, 23 comprises aplurality of independently-controllable parts 22 a, 23 a, whereby theheating resistors are heating element rows consisting of successiveparts 22 a, 23 a. The independently-controllable parts 22 a, 23 a of theheating resistors 22, 23 may be single elongated resistors, wherebythere are several successive elongated resistors in a row in the heatingresistor row. Each part 22 a, 23 a of FIG. 3 is shown as one, elongatedpiece for reasons of clarity, but a single part 22 a, 23 a, however,typically consists of several adjacent and separate resistor rods,whereby air can flow between them, at the same time effectively heatingthe air.

When the heating resistors 22, 23 consists of several, successiveindependently-controllable parts 22 a, 23 a, it is easier to manage thetransverse temperature profile of the glass sheet 6, whereby thetransverse temperature profiling of the glass sheet 6 can be carried outin a precise and managed way. So, by separately adjusting theindependently-controllable parts 22 a and 23 a in the heating resistors22 and 23, the temperature profile of the glass sheet 6 may easily andeffectively be adjusted in the transverse direction in relation to itsdirection of travel. In particular when the heating resistors 22 and 23are arranged in the blowing channels 7 and 14 they can be effectivelyused to adjust the temperature of the air blown onto the glass sheet 6.Further, when the blowing channels 7, 14 are substantially transverse inrelation to the direction of travel of the glass sheets, there will beno longitudinal discontinuity spots forming on the glass sheet in itsdirection of travel, but the temperature can be kept even in thetransverse direction.

FIG. 3 also schematically shows a control unit 24. The control unit 24is used to adjust the independently-adjustable parts 22 a and 23 a ofthe heating resistors 22, 23, which is illustrated by the reference mark25. The control unit 24 may be the same physical device as the controlunit 13, 20. Depending on the embodiment of the tempering furnace 1,each control unit may also control other devices of the temperingfurnace 1, such as the conveyor, or the moving of the top part 3 inrelation to the bottom part 4. For reasons of clarity, the supports,cabling, and similar items of the heating resistors 22, 23 or theirparts 22 a, 23 a are not shown in FIG. 3.

In the longitudinal direction of the glass sheets 6, in other words intheir direction of travel, the temperature profile of the glass sheets 6may be adjusted by adjusting the blowing force and/or the blowingdistance D of the successive blowing channels 7, 14 in the longitudinaldirection of the tempering furnace 1. The blowing force may be adjusted,for example, by independently adjusting blowers 9, 16 that are arrangedone after the other in the longitudinal direction of the temperingfurnace 1, making it possible to adjust the longitudinal profile of thetemperature, as regards the blowing force, at as many places as thereare independently-controllable blowers 9, 16 in the tempering furnace.

The temperature profile of the glass sheet 6 in its longitudinaldirection may also be adjusted, in addition to or instead of what wasdisclosed in the previous paragraph, by adjusting the power of theheating resistors 22, 23, or their parts 22 a, 23 a, placed one afterthe other in the longitudinal direction of the tempering furnace 1.

The blowing channels 7, 14 can also be divided into at least two partsin the transverse direction in relation to the direction of travel ofthe glass sheet 6, whereby the transverse temperature profile of theglass sheet 6 may also be adjusted by adjusting the blowing force, ifthe tempering furnace 1 is provided with separate blowers associatedwith said parts or with flow control valves that control the flow of theheating air to the corresponding parts in the blowing channel 7, 14.

In connection with FIG. 3, it is set forth that the transverse profileof the glass sheet temperature may be adjusted both from above and belowthe glass sheet. If desired, the transverse profile of the glass sheettemperature may be adjusted only from below or above. If the transverseprofile of the glass sheet temperature is only adjusted from above theglass sheet, for example, the heating means under the glass sheet may beformed simpler than what is described in FIG. 3. In such a case, theheating resistors 23 in the bottom part 4 do not necessarily needindependently-controllable parts 23 a, for example, but the heatingresistor 23 may be substantially of the length of the transversedirection of the tempering furnace 1.

In the above example, the blowing channels and heating resistors orheating resistor rows are arranged substantially transverse in relationto the direction of travel of the glass sheet. However, within the scopeof the solution put forth, also such embodiments of the temperingfurnace 1 are possible where the blowing channels and/or heatingresistors and/or heating resistor rows are arranged substantiallyparallel to the direction of travel of the glass sheet.

FIGS. 4, 5, and 6 show schematically a cross sectional detail of thetempering furnace 1, to be more precise, the interface between the toppart 3 and bottom part 4 of the frame structure 2 in the temperingfurnace 1. Each of the FIGS. 4, 5, and 6 present a possible embodimentfor said interface, as seen in FIGS. 1-3 on the side of the left wallstructure of the tempering furnace 1, whereby the left side of the wallstructure shown in each of the FIGS. 4, 5, and 6 represents the outsideof the tempering furnace 1 and the right side represents the inside ofthe tempering furnace 1. The purpose of the embodiment of each Figure isto seal the gap 21 that forms between the top part 3 and bottom part 4of the frame structure 2 of the tempering furnace 1 when the top part 3of the tempering furnace 1 is moved upward in relation to the bottompart 4.

In the embodiment according to FIG. 4, a flexible sealing element 26 isarranged in the tempering furnace 1 between a top part 3 surface 3 aaimed at the bottom part 4, and a bottom part 4 surface 4 a aimed at thetop part 3, the sealing element being fixed to said surfaces 3 a, 4 a.The sealing element 26 is arranged to extend on the side walls of thetempering furnace 1 over the entire length of the tempering furnace 1and also to the ends of the tempering furnace 1 to the extent that thesealing element 26 does not prevent the passage of the glass sheet 6into the tempering furnace 1 and out of it. The flexible sealing element26 allows the moving of the top part 3 in the vertical direction of thetempering furnace 1 in relation to the bottom part 4 without the gap 21,which forms between the top part 3 and the bottom part 4, opening to theoutside of the tempering furnace 1, the sealing element 26 thuspreventing the escape of the heating air out of the tempering furnace 1through said gap 21. The sealing element 26 may be of any insulatingmaterial that has adequate heat endurance.

In the embodiment of FIG. 5, a platelike sealing element 28 is fixed tothe outer surface of the wall structure in the top part 3 of thetempering furnace 1 with fasteners 27, such as screws, and arranged tobe aimed down towards the bottom part 4 of the tempering furnace 1 andto extend from the top part 3 down at a distance along the wallstructure of the bottom part 4 so that as the top part 3 moves up awayfrom the bottom part 4, said sealing element 28 is arranged to slidealong the outer surface of the wall structure of the bottom part 4,whereby the sealing element 28 closes the gap 21 forming between the toppart 3 and bottom part 4. The dimensions of the sealing element 28 inthe vertical direction of the tempering furnace 1 are designed so thatthe sealing element 28 covers the gap 21 forming between the top part 3and the bottom part 4 also in the operating situation of the temperingfurnace 1 where the distance between the top part 3 and bottom part 4 isat its maximum, in other words, at the maximum distance allowed by theadjustment range between the top part 3 and bottom part 4. The sealingelement 28 is arranged to extend on the side walls of the temperingfurnace 1 over the entire length of the tempering furnace 1 and also tothe ends of the tempering furnace 1 to the extent that the sealingelement 28 does not prevent the passage of the glass sheet 6 into thetempering furnace 1 and out of it. Similar sealing may be accomplished,for example, by a flap-like sealing element fixed in connection with thetop part 3.

In the embodiment according to FIG. 6, the surface 3 a in the top part3, aimed at the bottom part 4, has protrusions 29 aimed at the bottompart 4, and the surface 4 a in the bottom part 4, aimed at the top part3, has recesses 30 corresponding to said protrusions 29, saidprotrusions 29 being placed in said recesses 30 and being able to movein said recesses 30 in the vertical direction of the tempering furnace 1as the top part 3 of the tempering furnace 1 is moving in relation tothe bottom part 4. There may be one or more of said protrusions 29 andrecesses 30 in each wall structure, the quantity of the protrusions 29and recesses 30 being two in the embodiment of FIG. 6. The protrusions29 and recesses 30 are arranged to extend on the side walls of thetempering furnace 1 over the entire length of the tempering furnace 1and also at the ends of the tempering furnace 1 to the extent that thepassage of the glass sheet 6 into the tempering furnace 1 and out of itis not prevented. In the embodiment of FIG. 6, the protrusions 29 in thetop part 3 and the recesses 30 in the bottom part 4 form matchingsealing elements which close the gap 21 forming between the top part 3and the bottom part 4, and prevent the heating air from escaping fromthe tempering furnace 1 through said gap 21. The protrusions 29 and therecesses 30 in the vertical direction of the tempering furnace 1 aredesigned so that they cover the gap 21 forming between the top part 3and the bottom part 4 also in the operating situation of the temperingfurnace 1 where the distance between the surface 3 a in the top part 3,aimed at the bottom part 4, and the surface 4 a in the bottom part 4,aimed at the top part 3, is at its maximum. The embodiment according toFIG. 6 may also be implemented by arranging the protrusions 29 on thesurface 4 a in the bottom part 4, aimed at the top part 3, and byarranging the recesses 30 on the surface 3 a in the top part 3, aimed atthe bottom part 4.

It is obvious for a person skilled in the art that as the technologyadvances the basic idea of the invention may be implemented in variousways. The invention and its embodiments are thus not restricted to theabove-described examples but may vary within the scope of the claims.

The invention claimed is:
 1. A method for heating a glass sheet in aglass tempering furnace having a frame structure comprising a top partand a bottom part which may be moved in relation to each other in thevertical direction of the glass tempering furnace, at least one sealingelement adapted for sealing a gap that is formed between the top partand the bottom part when the top part is moved upward in relation to thebottom part, and at least one blowing channel arranged in its top part,the method comprising feeding the glass sheet to the glass temperingfurnace, and heating the glass sheet in the glass tempering furnace byat least blowing heating air on the top surface of the glass sheetthrough said at least one blowing channel, wherein the blowing distanceof the heating air from the top surface of the glass sheet is adjustedby changing the distance of the blowing channel from the top surface ofthe glass sheet, said distance being changed by changing the position ofthe top part of the glass tempering furnace in relation to its bottompart in the vertical direction of the glass tempering furnace.
 2. Amethod as claimed in claim 1, wherein the glass sheet is further heatedin the glass tempering furnace by blowing heating air on the bottomsurface of the glass sheet through at least one blowing channel arrangedin the bottom part of the glass tempering furnace.
 3. A method asclaimed in claim 1, wherein heating air is blown on the top surfaceand/or bottom surface of the glass sheet in a substantially transversedirection in relation to the direction of travel of the glass sheet inthe glass tempering furnace.
 4. A method as claimed in claim 1, whereinthe blowing force of the heating air is adjusted.
 5. A method as claimedin claim 1, wherein the glass sheet and/or the heating air blown on theglass sheet is further heated by at least one heating resistor arrangedin the top part and/or the bottom part of the glass tempering furnace.6. A method as claimed in claim 5, wherein the heating resistor isarranged substantially transverse in relation to the direction of travelof the glass sheet in the glass tempering furnace.
 7. A method asclaimed in claim 5, wherein the heating resistor comprises a pluralityof heating resistor parts placed one after the other, and the heating ofthe glass sheet and/or the heating of the heating air blown on the glasssheet is adjusted by the heating resistor by separately controlling thedifferent heating resistor parts of the heating resistor.
 8. A method asclaimed in claim 1, wherein said sealing element is a flexible sealingelement arranged between a top part surface aimed at the bottom part,and a bottom part surface aimed at the top part, the sealing elementbeing fixed to said top part and bottom part surfaces.
 9. A method asclaimed in claim 1, wherein said sealing element is a platelike sealingelement fixed to an outer surface of a wall structure in the top partwith fasteners, and arranged to be aimed down towards the bottom partand to extend from the top part down at a distance along a wallstructure of the bottom part so that as the top part moves up away fromthe bottom part, said sealing element is arranged to slide along theouter surface of the wall structure of the bottom part, whereby thesealing element closes the gap forming between the top part and thebottom part.
 10. A method as claimed in claim 1, wherein said sealingelement is one or more protrusions in a top part surface, theprotrusions and the top part surface aimed at the bottom part, and oneor more recesses in a bottom part surface, the bottom part surface aimedat the top part, said recesses corresponding to said protrusions, saidprotrusions being placed in said recesses and being able to move in saidrecesses in the vertical direction as the top part is moving in relationto the bottom part, the protrusions and recesses being arranged toextend on side walls of the tempering furnace over the entire length ofthe tempering furnace and at the ends of the tempering furnace to theextent that the passage of a glass sheet into the tempering furnace andout of it is not prevented.